Grades
Ferralium® 255
UNS S32550Ferralium® 255 is a proprietary super duplex stainless steel with high proof stress, PREN over 40 and excellent sub-zero toughness.
Alloy 32750
UNS S32750Alloy 32750 (2507) is a super duplex stainless steel with PREN over 40, listed in NACE MR0175 for sour service.
Alloy 32760
UNS S32760Alloy 32760 is a super duplex stainless steel with PREN over 40 and outstanding pitting, crevice and stress-corrosion resistance.
How the grades compare
At a glance: UNS number, defining property, typical applications and the forms we stock for each super duplex grade.
| Grade | UNS | Key property | Typical applications | Forms stocked |
|---|---|---|---|---|
| Ferralium® 255 | S32550 · F61 | The original super duplex; 25%Cr with 2% copper for superior acid and pitting resistance, and the highest strength (min 586 N/mm² proof) | Pumps, valves, shafts, wellhead and subsea, fasteners | Bar, Plate, Pipe & Tube |
| Alloy 32760 | S32760 · F55 | Super duplex (Zeron 100); 25%Cr with tungsten and copper, PREN >40 | Oil & gas, marine, process industry | Bar, Plate |
| Alloy 32750 | S32750 · F53 | Super duplex (2507); high Cr/Mo/N giving PREN >41 and CPT >50°C | Subsea, marine, chemical tankers, process plant | Bar |
Why specify super duplex stainless steel?
Super duplex combines the strength of ferritic and the toughness of austenitic stainless steels, with a Pitting Resistance Equivalent Number (PREN) above 40. Across the range you get:
- Outstanding resistance to pitting and crevice corrosion in seawater and chloride environments, with a Critical Pitting Temperature above 50°C
- PREN above 40, superior to austenitic and standard 22%Cr duplex grades in almost all corrosive media
- Roughly double the strength of austenitic stainless steels, allowing reduced section size, weight and cost
- Excellent resistance to sulphuric, nitric and phosphoric acids, enhanced by copper additions in Ferralium® 255
- Excellent ductility and impact strength from sub-zero to ambient temperatures
- High resistance to abrasion, erosion and cavitation
Material tools
PREN calculator
PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N. Guidance only; always confirm against the grade's datasheet before specifying.
Pitting & crevice resistance
Pitting & Crevice Resistance
Rank alloys by PREN and get an indicative pitting temperature for a chloride service. Pick a grade and a service temperature to see whether it has margin against pitting, or compare the range in the table below. A first-pass screen for chloride and seawater service.
| Grade | UNS | PREN | Indicative CPT | Relative pitting resistance |
|---|
Indicative screen, not a certified result. PREN ranks pitting resistance; the pitting temperature here is estimated from PREN (CPT ≈ 2.5 × PREN - 50), consistent with Langley's "PREN >40, CPT exceeding 50°C" for super duplex. Real critical pitting/crevice temperature depends on test method, product form, surface finish and service chemistry. Crevice corrosion starts well below the pitting temperature - allow a margin at gaskets, deposits and flange faces. Use certified data for design.
Sour-service screen (NACE MR0175)
Sour-Service Screen
Work out whether a service is sour from the H2S partial pressure, and see each alloy's NACE MR0175 / ISO 15156 status and hardness limit. A first-pass screen for oil & gas material selection - the full environmental envelope for a grade is set in ISO 15156-3 Annex A.
Candidate grade
| Grade | Family | NACE status | Hardness limit |
|---|
First-pass screen only. This shows whether the service is sour (from H2S partial pressure) and each material group's NACE MR0175 hardness limit and general suitability. It does not reproduce the full environmental windows (maximum H2S partial pressure, temperature, chloride, in-situ pH per grade) - those are in ISO 15156-3 Annex A and must be checked there for the specific grade, product form and condition. Qualification also depends on heat treatment, cold work and welding.
Seawater velocity limits
Seawater Velocity Limits
Check a seawater flow velocity against the maximum design velocity for copper and copper-nickel alloys, to avoid impingement (erosion-corrosion) attack. Enter a velocity directly, or work it out from flow rate and pipe bore. Design guidance from CDA / Nickel Institute and BSMA 18.
| Alloy | UNS | Design velocity | BSMA 18 pipe |
|---|
Design guidance, not a guarantee. Maximum velocities avoid impingement/erosion-corrosion in clean seawater. Limits reduce at small bores (BSMA 18 pipe values apply above ~100 mm bore, reduced below). Local turbulence at bends and fittings, partial blockage, entrained air or sand, and higher temperature all lower the safe velocity. Confirm against the project specification and the product's own data.
Applications
How do you machine super duplex stainless steel?
Super duplex is high-strength and work-hardening, so it machines more like a nickel alloy than a standard stainless. Use rigid set-ups and sharp, positive-rake carbide tooling, keep the cut continuously engaged to avoid glazing the surface, and run moderate speeds with a firm, constant feed and flood coolant.
Because the alloy is strong and abrasive, tool wear is higher than for austenitic grades; robust tooling and secure workholding pay off. Enhanced-machinability variants are available where higher metal-removal rates are needed.
Technical resources
Common questions
What are super duplex stainless steels?
'Super duplex' describes highly alloyed duplex stainless steel with a Pitting Resistance Equivalent Number (PREN) above 40. Its mixed austenitic-ferritic microstructure delivers both high strength and excellent corrosion resistance. Langley Alloys developed the first commercial super duplex, Ferralium® 255, in the late 1960s.
What is the difference between duplex and super duplex?
Super duplex is based on around 25% chromium versus about 22% for standard duplex. The higher chromium raises the PREN from around 34 to above 40, giving superior pitting corrosion resistance across a broader range of environments.
What is Ferralium®?
Ferralium® is a trademark of Langley Alloys and the first commercial super duplex stainless steel, launched in 1969 after a 1967 patent. Ferralium® 255-SD50 (UNS S32550) is the only super duplex to achieve a minimum yield strength above 85 ksi, and its higher copper content gives superior corrosion performance.
What is Alloy 32760 (Zeron 100)?
Alloy 32760 (UNS S32760, also known as Zeron 100, F55) is a 25% chromium super duplex developed in the 1980s, with tungsten and copper additions. Langley stocks it as bar up to 16 inch diameter.
What is the difference between the various super duplex alloys?
Ferralium® 255 (S32550) is the original, with the highest strength and extra copper for acid resistance. Alloy 32760 (S32760, Zeron 100) and Alloy 32750 (S32750, 2507) are later 25%Cr grades with slightly different balances; all exceed PREN 40.
What is PREN?
The Pitting Resistance Equivalent Number estimates resistance to pitting corrosion, calculated as %Cr + 3.3 × %Mo + 16 × %N. Super duplex grades exceed 40, indicating outstanding pitting resistance.
History of super duplex stainless steel
Super duplex stainless steels take the duplex idea further. By raising chromium to around 25% and adding molybdenum, nitrogen and, in some grades, tungsten and copper, they reach a pitting resistance equivalent number above 40, giving outstanding resistance to pitting and crevice corrosion in hot, chloride-rich seawater and sour service. Langley Alloys' own Ferralium® 255 was the first commercial super duplex stainless steel, introduced in 1967.
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The class came into its own in the 1980s, when North Sea oil and gas needed alloys that could survive seawater, high chlorides and hydrogen sulphide at pressure while allowing lighter, thinner sections than the nickel alloys of the day. Alongside Ferralium® 255 (S32550), grades such as 2507 (S32750) and Zeron 100 (S32760) became the workhorses of subsea and topside equipment, pumps, valves and wellhead components.
One of the most visible uses was the 1986 restoration of the Statue of Liberty. Ferralium® 255 was chosen for the rigid flat bars spanning the internal armature and the secondary frame, valued for its strength and its low galvanic reaction with the statue's copper skin, a rare case where a super duplex stainless steel is quite literally helping hold up an icon.
Ferralium® 255, the first commercial super duplex, invented by Langley in 1967, still sets the benchmark for strength and corrosion resistance in seawater and acid service.
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